ZEKE Photoelectron Spectroscopy of p-Fluorophenol - American

Sep 10, 2014 - ZEKE Photoelectron Spectroscopy of p‑Fluorophenol···H2S/H2O. Complexes and Dissociation Energy Measurement Using the Birge−...
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ZEKE Photoelectron Spectroscopy of p‑Fluorophenol···H2S/H2O Complexes and Dissociation Energy Measurement Using the Birge− Sponer Extrapolation Method Surjendu Bhattacharyya† and Sanjay Wategaonkar* Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India S Supporting Information *

ABSTRACT: In this work we have shown that the Birge−Sponer extrapolation method can be successfully used to determine the dissociation energies (D0) of noncovalently bound complexes. The O−H···S hydrogen-bonding interaction in the cationic state of the p-fluorophenol···H2S complex was characterized using zero kinetic energy (ZEKE) photoelectron spectroscopy. This is the first ZEKE report on the O−H···S hydrogen-bonding interaction. The adiabatic ionization energy (AIE) of the complex was determined as 65 542 cm−1. Various intermolecular and intramolecular vibrational modes of the cation were assigned. A long progression was observed in the intermolecular stretching mode (σ) of the complex with significant anharmonicity along this mode. The anharmonicity information was used to estimate the dissociation energy (D0) in the cationic state using the Birge−Sponer extrapolation method. The D0 was estimated as 9.72 ± 1.05 kcal mol−1. The ZEKE photoelectron spectra of analogous complex FLP···H2O was also recorded for the sake of comparison. The AIE was determined as 64 082 cm−1. The intermolecular stretching mode in this system, however, was found to be quite harmonic, unlike that in the H2S complex. The dissociation energies of both the complexes, along with those of a few benchmark systems, such as phenol···H2O and indole···benzene complexes, were computed at various levels of theory such as MP2 at the complete basis set limit, ωB97X-D, and CCSD(T). It was found that only the ωB97X-D level values were in excellent agreement with the experimental results for the benchmark systems for the ground as well as the cationic states. The dissociation energy of the (FLP···H2S)+ complex determined by the Birge−Sponer extrapolation was about ∼18% lower than that computed at the ωB97X-D level.

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INTRODUCTION Hydrogen-bonded (H-bonded) complexes are a special class of complexes bound by noncovalent interactions.1 The interaction strength or the dissociation energies (D0) of such complexes are strong enough to influence the structural/physical properties of substances in the condensed phase and at the same time they are weak enough so that they can be manipulated at standard temperature and pressure (STP). This has been evident in chemistry by their influence in affecting reaction mechanisms as well as the rates and catalytic abilities.2−4 In biological systems, these are important in determining the structures of proteins, DNAs, the protein−DNA interactions, enzyme activities, etc.5 In the last three decades a lot of investigations have been reported on the microscopic properties of the H-bonded systems that include both conventional as well as unconventional hydrogen bonds.2,5−9 The IR spectroscopic data on the shifts in the stretching frequency of the hydrogen-bond (H-bond)-donating X−H group, where X is an atom to which the hydrogen atom is bound covalently, have been extensively used in determining the sites and structures of H-bonded complexes. The strength of hydrogen-bonding (H-bonding) interaction, which is a very important piece of information is, however, not always easy to determine experimentally. More often than not, © 2014 American Chemical Society

the IR data on the X−H stretching frequency are used to infer the strength of the H-bonded complex, depicted by the X−H···Y interaction, where X and Y are the H-bond donor and acceptor atoms, respectively. In a limited number of cases the D0 values of the H-bonded complexes have been determined experimentally. Mons et al. pioneered the dissociative photoionization technique using two-color resonantly enhanced twophoton ionization and determined the dissociation energies of H-bonded complexes of phenol and indole.10−12 The D0 values are available for a few other H-bonded complexes that were determined by using stimulated emission pumping-resonant two-photon ionization spectroscopy,13 zero kinetic energy photoelectron (ZEKE) spectroscopy, mass analyzed threshold ionization (MATI) spectroscopy, or velocity map imaging (VMI).14−17 ZEKE spectroscopy1 is a state-of-the-art technique to determine the adiabatic ionization energy (AIE) accurately and the ro-vibrational levels of a molecule in the cationic state. With this technique, the D0 of the complexes has been determined by extending the spectra up to the dissociation limit in the cationic state. Wider application of the aforementioned Received: June 1, 2014 Revised: September 7, 2014 Published: September 10, 2014 9386

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used as an excitation source (S1−S0), and a Quantel TDL90 dye laser pumped by a Brilliant-B Q-Switched Nd:YAG laser (fwhm ∼6 ns, 10 Hz repetition rate) was used as the tunable ionization (D0−S1) laser. The ionization laser was scanned from 29 000 to 33 000 cm−1 by changing several sets of dyes such as LDS698, LDS698+DCM, DCM, DCM+R640, and R610. Temporal synchronization of the lasers was done by SRS Delay generator (DG645). UV beams were set in a copropagating geometry and spatially overlapped by a 50 cm focal length biconvex quartz lens. For the ZEKE pulsed field ionization spectroscopy the excitation laser (pump laser) excited the monomer/complex to a stable intermediate state and the second laser (probe laser) took it to the Rydberg manifold. The electron is so loosely attached to the cationic core in a Rydberg state that the core holds all the characteristic properties of the cation. A mild perturbation such as weak electric fields can cause field ionization of the Rydberg neutrals. Therefore, by tuning the probe laser, the Rydberg manifolds associated with various vibronic levels of the cation were identified, which essentially provided the information about the ro-vibrational levels of the associated cationic state. At the instant of interaction of the two laser pulses with the cold molecular beam, the bottom and middle grids were held at a spoil field (E) of 720 mV/cm to separate the prompt electrons from the Rydberg neutrals. The bottom grid was pulsed to field ionize the Rydberg states after a delay of about 500 ns. Absolute wavelength calibration for the dye lasers was performed using a Fe−Ne hollow cathode lamp using Optogalvanic method. The uncertainty in the UV wavelengths was less than 2 cm−1. The typical pulse energies were ∼50 μJ for the excitation laser and ∼1−2 mJ for the ionization laser. All the ZEKE spectra reported in this report are UV power normalized. For our experiment, FLP (99%), was bought from SigmaAldrich. Zero grade H2S (99.5%) was locally purchased from Ultra-Pure Gases (I) Pvt. Ltd. Millipore water purified by Elix 5 was used for preparation of the water premix. FLP was heated to 75 °C to produce enough vapor pressure for the best signalto-noise ratio for the monomer as well for the complexes. The 1:1 complexes were prepared by coexpanding the FLP vapor with 2−5% premix in helium of the corresponding solvent (H2O/H2S) under a stagnation pressure of 1−3 kg cm−2. The typical working pressure in the expansion chamber was 5 × 10−5 Torr, and that in the ionization chamber was 2 × 10−6 Torr. Premix concentration, stagnation pressure, laser power, and wavelength of the ionization laser were critically optimized to obtain the best possible signal-to-noise ratio. Computational Methods. It has been shown in our earlier report18 that the most stable conformer of the FLP−H2O and FLP−H2S complexes is the one in which the phenolic O−H acts as the H-bond donor and H2O/H2S acts as an acceptor. Therefore, the structures of the O−H···A (A = O/S) conformer of both the complexes were optimized at three different levels, namely, MP2, B3LYP, and ωB97X-D on a counterpoisecorrected surface (cp) using the aug-cc-pVDZ basis set. They are represented as cp-MP2, cp-B3LYP, and cp-ωB97X-D. These methods have been chosen for different reasons; MP2 takes care of the electron correlation,25,26 which is quite important for weak H-bonding interaction, whereas B3LYP is computationally less expensive and yields reasonably trustworthy vibrational frequencies under harmonic approximation.27 However, the B3LYP functional does not account for dispersion interaction. Therefore, another DFT functional,

techniques, however, is hindered by the experimental challenges involved in these measurements. In recent past we have characterized several size-selected O−H···S (S = sulfur atom of acceptor solvent molecule) Hbonded complexes in ground and first excited states.18,19 The abundance of structural information on the X−H···S (X = covalently bound atom or group of atoms) type noncovalent interactions in the crystallographic database of proteins and many organic molecules motivated the spectroscopic investigation of this interaction.20,21 The dissociation energies of the S-centered complex reported in these works have been obtained computationally. It has been shown that in a few cases the O−H···S interactions can be as strong as the O−H···O interactions depending on the proton affinity of the acceptor.18 Further, in the ground state, the O−H···S H-bonds are dominated by dispersion interaction, unlike the conventional O−H···O H-bonding interactions, which are dominated by electrostatic interaction.18,19 However, the O−H···S Hbonded complexes have never been explored in cationic state where the electrostatic interaction (charge−induced-dipole) is expected to be dominant. It is important to find out the characteristic changes in properties such as the red shift in the X−H stretching frequency, intermolecular stretching frequency, dissociation energy, etc. of the predominantly dispersionstabilized O−H···S H-bonded complex when it is ionized to the cationic state. The study of the cationic state is also helpful in obtaining useful information about ionic clusters and ion− solvent interactions.1 We have carried out ZEKE spectroscopic investigations on the FLP−H2S complex in the cationic state. The AIE and the intramolecular and intermolecular vibrational levels of the complex in the cationic state were determined. The most salient feature of the ZEKE spectra was a long progression in the intermolecular stretching mode, σ. We show that the anharmonicity observed in the potential along this normal mode can be used to estimate the dissociation energy, D0, of the complex using the Birge−Sponer (BS) extrapolation22 method, which is the first demonstration of its kind for a Hbonded polyatomic system. Using very high level computations, we validate the application of the BS method to this large polyatomic weakly H-bonded system. We also present the ZEKE spectroscopic data on the analogous system, namely, the p-fluorophenol−H2O (FLP−H2O) complex, for the sake of comparison. Unlike the (FLP−H2S)+ complex, the progression in the intermolecular stretching mode was found be to almost harmonic. The possible reasons for this difference will be discussed.

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METHODS Experimental Methods. A detailed description of the experimental apparatus has been given in our earlier works.23 The TOFMS used for the present experiments was Wiley− McLaren type24 with a 20 cm long flight tube. During acquisition of the photoionization efficiency (PIE) curve all three grids are kept grounded at the instant of photoionization. After a delay of 500 ns, the bottom and middle grids were pulsed to +2900 and +2100 V, respectively, using fast highvoltage transistor switches (15 ns rise time) and the total ion current was measured as a function of the ionization laser wavelength. Intensity of the ion current vs the sum of the twophoton energies constructed the PIE curve. A Sirah Cobra Stretch-LG-18 dye laser pumped by a Nd3+:YAG laser (SpectraPhysics Quanta-Ray, PRO-Series, 10 Hz, ∼10 ns fwhm) was 9387

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namely, ωB97X-D, was chosen because this includes the atom− atom dispersion corrections and has been reported to have yielded satisfactory results.18,28,29 The other variants of the B97 functional such as B97D and RI-B97D have been also reported to produce satisfactory results.30−33 Frequency calculations were done at each level for all conformers to ensure that the optimized structures were indeed the minima on the potential energy surface (PES). The D0 values were calculated after adding the zero point energy difference (ΔZPE). Because the optimizations were performed on the counterpoise-corrected surface, separate correction for basis set superposition error (BSSE) was not required. All the calculations were performed for the ground state as well as the cationic state using Gaussian 09 suite of programs without any constraints.34 To refine the D0 values further, the following exercises were carried out. In addition to the complexes under investigation the D0 values were computed for a couple of benchmark systems, namely, the phenol−water (PHE−H2O) and the indole−benzene (IND− BEN) at all the three levels on the counterpoise-corrected surface. Additionally, the D0 of the ground state was extrapolated to complete basis set (CBS) limit by the extrapolation formula of Helgaker et al.35 E CBS ≈

also shown in Figure S1 (trace b, Supporting Information). The AIE of FLP monomer was determined as 68 567 ± 2 cm−1 in the presence of the spoil field of 720 mV/cm. The field-free AIE was obtained by extrapolation to zero field according to the function δ(AIE) = 3.94(E)1/2, where E is the spoil field in V/cm as mentioned in the literature.42 The field-free AIE of FLP was thus found to be 68 570 cm−1, which agreed well with the previously reported value by Tzeng and co-workers,42 who determined it using MATI spectroscopy.42 The other transitions observed at 68 928 (+361) and 69 018 cm−1 (+451) were ascribed to intramolecular modes 151 and 6a1, respectively. The numbers in parentheses refer to the energy relative to the AIE and have been given next to each transition. Varsanyi’s nomenclature was adopted for the assignments of the observed normal modes of the monomer and the complexes.43 These assignments are in agreement with the MATI data available for FLP.42 ZEKE Spectroscopy of FLP−H2S. The PIE curve for the FLP−H2S complex determined by exciting the 000 transition in the S1 state is shown in Figure S2 (trace a, Supporting Information). This very slow rising PIE curve, which was ∼500 cm−1 wide, made it very difficult to predict the ionization threshold. Figure S2 (trace b, Supporting Information) displays the ZEKE spectrum of FLP−H2S via the S1 000 excitation. The field-corrected AIE of FLP−H2S was determined as 65 542 ± 2 cm−1, which was 3028 cm−1 red-shifted with respect to that of the monomer. This is the first experimental report of AIE of any sulfur-bound complex in the gas phase. An extended ZEKE spectrum of FLP−H2S via the S1 000 intermediate state beyond 2400 cm−1 above its AIE is displayed in Figure 1a. The

X3E(X ) − (X − 1)3 E(X − 1) X3 − (X − 1)3

X = 3 for pVTZ and 4 for pVQZ

For TZ-DZ extrapolation, the single point energy including BSSE was calculated for the cp-MP2/aug-cc-pVDZ optimized geometry at the cp-MP2/aug-cc-pVTZ level and ΔZPE was calculated at the cp-MP2/aug-cc-pVDZ level. For QZ-TZ extrapolation, single point energy including BSSE was calculated for the cp-MP2/aug-cc-pVTZ optimized geometry at the cp-MP2/aug cc-pVQZ level and ΔZPE was calculated at the cp-MP2/aug-cc-pVTZ level. The D0 was also calculated at the CCSD(T) level by computing the single-point energy (including BSSE) for the cp-MP2/aug-cc-pVDZ optimized geometry using the same basis. The ΔZPE for CCSD(T) calculation was calculated at the cp-MP2/aug-cc-pVDZ level. The topology of the electron density was analyzed by the AIM2000 program.37 The electron density (ρH‑‑‑Y) and Laplacian of the electron density (∇2ρH‑‑‑Y) at bond critical point (BCP) upon H-bond formation were calculated at the MP2/aug-cc-pVDZ level from the wave functions computed at the cp-MP2/aug-cc-pVDZ level.38 Natural bonding orbital analysis (NBO) was performed using the NBO 5.9 program to generate the natural orbitals of the system and to calculate the secondary orbital interactions (E(2)(i‑j)) between the donor and acceptor of the H-bonded complexes.39 Localized molecular orbital energy decomposition analysis (LMOEDA)40 was performed for the complexes in S0 and D0 states in GAMESS, USA.41

Figure 1. ZEKE spectra of FLP−H2S recorded via (a) S1 000 and (b) S1 σ10 intermediate levels. The intramolecular fundamental vibrational modes are shown by the blue dotted lines, and the intermolecular vibrational stretching mode transitions and its overtones are shown by the red dotted lines.

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EXPERIMENTAL RESULTS ZEKE Spectroscopy of the FLP Monomer. The S1−S0 electronic excitation spectra of FLP and its 1:1 complexes with H2O/H2S have been reported in our earlier work.18 The PIE curve of FLP (Figure S1, trace a, Supporting Information) was recorded by fixing the excitation laser at the S1−S0 band origin at 35 119 cm−1 while scanning the ionization laser. The onset of the ionization was found at around 68 550 cm−1. The rising edge of the PIE curve was ∼50 cm−1 wide. The ZEKE spectrum of FLP recorded via the S1−S0 band origin (S1 000) transition is

intention was to scan up to the dissociation limit in the cationic state to determine the D0. However, the S/N became so poor beyond 2000 cm−1 that it was realized to be a futile exercise. The remarkable structure in the ZEKE spectrum beneath the slow rising PIE curve is due to a long progression in the intermolecular stretching vibration up to ν′ = 5 at 146, 288, 428, 567, and 701 cm−1, respectively, with the strongest 9388

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transition being σ3 at 428 cm−1 and a relatively weaker transition σ5 at 701 cm−1. The other transitions observed at 115, 447, 658, 836, 938, 1156, and 1638 cm−1 were assigned to the intramolecular fundamental modes 111, 6a1, 6b1, 11, 17b1, 18b1, and 19b1, respectively. The progression in the σ mode was observed in combination with most of the fundamental normal modes (see the Supporting Information for details). The intensity distribution among the progression members of the combination bands with all the intramolecular modes followed the same pattern as that at the origin. Figure 1 (trace b) presents the ZEKE spectrum of FLP−H2S via the σ10 transition in the S1 state. The lowest energy transition, i.e., the AIE, was observed at 65 539 cm−1, which was one of the intense transitions. The intensity distribution among the progression members in this spectrum and the one recorded via the S1 000 transition was quite different. Although the σ1, σ4, σ5, and σ6 transitions observed at 145, 566, 700, and 834 cm−1, respectively, were quite strong, the intensity of the σ2 and σ3 transitions at 289 and 432 cm−1, respectively, diminished to a great extent. The σ6 transition overlapped with the 11 transition, gaining unusually high intensity compared to that in ZEKE spectra when recorded via the S1 000 transition. The detailed assignments of all the transitions observed in the ZEKE spectra recorded via different modes are provided in Table S1 (Supporting Information), and the normal modes observed in the spectra are listed in Table 1. ZEKE Spectroscopy of FLP−H2O. Figure S3 (trace a, Supporting Information) depicts the PIE curve for the FLP− H2O complex. Unlike the previous case the PIE curve of FLP− H2O has a sharp onset at 64080 cm−1 with a staircase like structure; the rising edge of each step was ∼20−40 cm−1 wide.

When the ZEKE spectrum was recorded via the S1 000 band origin, it was observed (Figure S3, trace b, Supporting Information) that underneath every step of the rising edge in the PIE curve there was a strong ZEKE peak. There were even weaker ZEKE features that were impossible to identify from the continuum of the PIE curve. The field-corrected AIE of FLP− H2O was determined to be 64 082 ± 2 cm−1, which was 4488 cm−1 red-shifted with respect to the monomer. This was comparable with the red shift reported for the PHE−H2O complex of 4602 cm−1.44 The intermolecular stretching vibrational mode (σ1) was the most intense observed transition at 235 cm−1 with its overtones at 470 and 703 cm−1. A much weaker transition at 385 cm−1 was assigned as 151. Modes 6a1 and 6b1 were identified at 456 and 673 cm−1, respectively. The assignments of various inter and intramolecular modes were based on ZEKE and MATI data available for various phenols and PHE−H2O complex in the literature.42,44−46 The ZEKE spectra up to 3500 cm−1 above the ionization potential are displayed in Figure 2 (trace a). The ZEKE scan

Table 1. Normal Mode Assignments in the ZEKE Spectra and Computed Normal Modes for the Complex Cationsa experimental intermediate level in the S1 state 00

235 385 456 673 836 885 1207 1634 115 146 447 658 836 938 1156 1638 a

σ1

189 235

674 837 891 1639 117 145 452 834 1159 1195 1637

6a1

computational cp-B3LYP

cp-MP2

cp-ωB97X-D

FLP−H2O in the Cationic State 153 136 137 135 166 182 168 236 240 212 237 372 368 377 450 458 465 460 676 614 591 617 835 847 889 856 895 942 830 949 1168 1192 1174 1519 1696 1548 FLP−H2S in the Cationic State 136 138 136 146 131 147 454 464 457 611 586 616 846 890 856 877 963 887 1120 1163 1132 1166 1184 1178 1522 1699 1548

Figure 2. ZEKE spectra of FLP−H2O recorded via (a) S1 000, (b) S1 σ10, and (c) S1 6a01 intermediates. The intramolecular fundamental vibrational modes are shown by the blue dotted lines, and the intermolecular vibrational stretching mode transitions and its overtones are shown by the red dotted lines.

mode assignment 111 τ1 σ1 151 6a1 6b1 11 9b2 9a1 19b1

could not be continued further in a bid to obtain the dissociation limit because of the poor S/N ratio beyond 2500 cm−1. Other intramolecular fundamental modes were found at 836, 1207, and 1634 cm−1, which were assigned as 11, 9a1, and 19b1, respectively. A progression in the intermolecular stretching mode σ was observed up to ν′ = 5 at 235, 470, 703, 942, and 1176 cm−1. No significant anharmonicity was observed in the intermolecular stretching mode so far within the error limit. The observed frequencies of the intramolecular normal modes are listed in Table 1, which were in good agreement with those computed for the cationic state as listed in the same table. The ZEKE spectrum of FLP−H2O recorded via the S1 σ10 intermediate level is shown in Figure 2 (trace b). The signal to noise ratio became poor beyond 1000 cm−1, and the intensity pattern of the transitions was different from that observed with excitation via the S1 000 transition. This suggested that the Franck−Condon (FC) factors were very different for these two

111 σ1 6a1 6b1 11/σ6 17b1 18b1 9a1 19b1

All the numbers are in cm−1. 9389

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which is the upper most bound vibrational level. The dissociation energy can be calculated from the knowledge of the vmax value. The level spacing, Δε, between the observed successive members of the progression in the σ mode were used for estimating the D0 of FLP−H2S in cationic state through BS extrapolation assuming the complex was pseudodiatomic (vide infra). Figure 3 shows the BS linear extrapolation of the Δε vs ν

cases. The lowest energy transition, the AIE, was observed at 64 076 cm−1, which was the most intense transition. The second most intense transition observed at +235 cm−1 was assigned to σ1. Although the σ2 transition could not be observed at all, the overtones σ3, σ4, and σ5 were identified at 708, 946, and 1181 cm−1, respectively. This intensity distribution pattern among the members of the progression is completely different from the one observed in Figure 2a. The detailed assignments of other features are given in the Supporting Information. The trace c in Figure 2 shows the ZEKE spectrum of FLP− H2O by exciting 6a10 in the S1 state. The S/N was poorer than the other two ZEKE spectra. Also the intensity profiles of the transitions were also different indicating substantial changes in the FC overlap between vibronic levels of the cationic state and various intermediate levels in the S1 state used to record the ZEKE spectra. The AIE was observed at 64 077 cm−1, which was very weak in intensity due to the poor overlap between the S1 6a10 level and the cation origin. The transition due to intermolecular stretching mode was found at 236 cm−1. One additional new peak in this spectrum, although very weak, identified at 153 cm−1 was assigned to 111. The complete assignments of the transitions observed in the ZEKE spectra of FLP−H2O via all three intermediate states are provided in Table S1 (Supporting Information). Dissociation Energy of the FLP−H2S Complex Using Birge−Sponer Extrapolation. The dominant feature of the ZEKE spectrum of FLP−H2S measured via the S1 000 transition was the progression along the intermolecular H-bond stretching mode, as shown by the red dotted lines in Figure 1. The progressions were observed at the band origin as well as in combination with all the intramolecular vibrational modes. A similar observation was made on the ZEKE spectrum measured by exciting the σ1 transition in the S1 state. The fundamental transition of the intermolecular stretching mode, σ1, was identified at 145 cm−1. The energy level spacings between the successive members, i.e., the Δε values, observed in all these progressions were averaged and found to be 145, 144, 141, 138, and 133 cm−1. The decrease in the level spacing in the successive members of the progression indicated a fair amount of anharmonicity along this normal mode. Spectroscopically, a dissociation limit for a bound system can be determined when a convergence limit of bands is observed in either emission or absorption. These bands could be the electronic bands of a neutral system approaching an ionization limit or the vibronic bands within a given state. Birge and Sponer have suggested22 an extrapolation to the convergence limit from the few observed bands for determining the dissociation limit, the assumptions being that, at the limit the energy separation between two successive levels becomes zero and one is fully aware of the dissociation products and their respective states. The details of the Birge−Sponer (BS) extrapolation are described below. With the expression for the εv levels that excludes all the terms beyond the quadratic term, the energy separation between successive levels (in cm−1) is given by

Figure 3. Birge−Sponer linear extrapolation plot for the FLP−H2S in the cationic state. Correlation coeff = 0.9570, intercept = 146.20 ± 0.86 cm−1, and Slope = −3.21 ± 0.34.

plot to the convergence limit with the experimentally determined five data points for σ (v = 0 to v′ = 1−5). The estimated values of ωe, ωexe, and νmax from the slope and intercept of the linear extrapolation (Figure 3) were 149.41 cm−1, 1.6062 cm−1, and 45, respectively. The dissociation energy, D0, using the BS extrapolation method in the cationic state was thus estimated as 9.72 ± 1.05 kcal mol−1.

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COMPUTATIONAL RESULTS Table 2 lists the dissociation energies of the optimized structures computed at various levels, as mentioned in the Methods, for both the complexes as well as for the two benchmark systems, PHE···H2O and IND···BEN complexes. The ground state D0 values for the PHE···H2O complex were 4.67, 5.18, 4.06, and 4.57 kcal mol−1 at the cp-MP2, cp-ωB97XD, cp-B3LYP, and CCSD(T) levels, respectively. The dissociation energies at the CBS limit using the MP2/TZ-DZ and QZ-TZ extrapolations were 5.11 and 5.25 kcal mol−1, respectively. The QZ-TZ, TZ-DZ, and the cp-ωB97X-D values were consistent with each other and agreed the best with the experimental value of 5.60 kcal mol−1.10 For the IND···BEN complex the D0 value computed at the cp-ωB97X-D level was in excellent agreement with the experimentally reported value.48 From the consistency point of view similar observations were made for the FLP−H2O and FLP−H2S complexes, although there is no experimental measurement for these complexes. The QZ-TZ, TZ-DZ, and the cp-ωB97X-D values for the FLP−H2O complex were 5.58, 5.39, and 5.36 kcal mol−1 and those for the FLP−H2S complex were 3.50, 3.39, and 3.37 kcal mol−1, respectively.

Δε = εν + 1 − εν = [−2ωexe]ν + [ωe − 2ωexe]

where ωe is the oscillation frequency in cm−1 and ωexe is the anharmonicity constant. Clearly, Δε varies linearly with ν. From the slope and intercept of the Δε vs ν plot, one can obtain the ωe and ωexe values. By equating the left-hand side of the above equation to zero, which is an assumption at the dissociation threshold, one can determine the value of vmax, 9390

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Table 2. Summary of Experimentally Determined and Computed Dissociation Energies (D0)a phenol−water method EXPT cp-B3LYP cp-ωB97X-D cp-MP2 CCSD(T)e cp-MP2 (TZ)f MP2 CBS (TZ-DZ)g MP2 CBS (QZ-TZ)g

gr state 5.60 4.06 5.18 4.67 4.57 4.96 5.11 5.25

b

ΔAIE

b

b

18.76 16.99 17.91 15.07 16.54 N/A N/A N/A

p-fluorophenol−H2O

indole−benzene

cation

13.16 12.93 12.73 10.40 11.97 N/A N/A N/A

gr state c

5.20 1.21 5.29 N/A N/A N/A N/A N/A

cation

ΔAIE

c

c

13.10 N/A 12.43 N/A N/A N/A N/A N/A

7.90 N/A 7.14 N/A N/A N/A N/A N/A

gr state N/A 4.41 5.36 4.99 4.90 5.28 5.39 5.58

cation N/A 17.10 18.02 14.27 16.59 N/A N/A N/A

ΔAIE

p-fluorophenol−H2S gr. state

cation

N/A 1.94 3.37 3.01 2.77 3.29 3.39 3.50

9.72 ± 1.05 10.92 11.92 9.29 10.87 N/A N/A N/A

12.83 12.69 12.66 9.28 11.69 N/A N/A N/A

ΔAIE d

8.66 8.98 8.55 6.28 8.10 N/A N/A N/A

The computed D0 values are corrected for the ΔZPE and BSSE. All the numbers are in kcal mol−1. bReference 10. cReference 48. dFrom Birge− Sponer extrapolation. eThe D0 at CCSD(T)/aug-cc-pVDZ level was calculated by computing single point energy (including BSSE) for the cp-MP2/ aug-cc-pVDZ optimized geometry. The ΔZPE was calculated at cp-MP2/aug-cc-pVDZ level. fThe D0 (includes ΔZPE and BSSE) computed by optimizing geometry and calculating frequency at cp-MP2/aug-cc-pVTZ level. gThe D0 estimated to CBS limit by the two-point extrapolation method. For TZ-DZ extrapolation, the single-point energy including BSSE was calculated for the cp-MP2/aug-cc-pVDZ optimized geometry at the cp-MP2/aug-cc-pVTZ level and ΔZPE was calculated at the cp-MP2/aug-cc-pVDZ level. For QZ-TZ extrapolation, the single-point energy including BSSE was calculated for the cp-MP2/aug-cc-pVTZ optimized geometry at the cp-MP2/aug-cc-pVQZ level and ΔZPE was calculated at the cp-MP2/aug-cc-pVTZ level. a

Table 3. Summary of ab Initio Computations, QTAIM and NBO Analysesa molecules computed parameters dH···A/Å RO···A/Å ΔrO−H/Å θ/deg ψ/deg ΔνO−H/cm−1 σ1/ cm−1 σ1/cm−1 (exptl) ρO−H···A/au keff(σ1)/(N/M)c keff(σ1)/(N/M)c (exptl) ∇2ρO−H···A/au ρC−H···A/au ∇2ρC−H···A/au E(2)(i-j)/kcal mol−1 LP (1) → σ*(O−H) LP (2) → σ*(O−H)

FLP−H2O

FLP−H2S

(FLP−H2O)+

(FLP−H2S)+

1.9205 (1.8890) 2.8952 (2.8578) 0.0079 (0.0115) 176.8 134.9 156 147 (147)

2.5184 (2.4613) 3.4811 (3.4173) 0.0056 (0.0075) 170.0 92.4 118 84 (94) 88b 0.0136 10.95 11.90b 0.0364

1.6832 (1.6021) 2.6826 (2.6084) 0.0264 (0.0372) 178.4 139.4 540 212 (237) 235 0.0415 41.04 50.47 0.1626 0.0070 0.0284

2.2246 (2.1409) 3.2184 (3.1401) 0.0231 (0.0329) 175.1 97.3 490 131 (147) 145 0.0266 26.33 32.32 0.0623 0.0053 0.0170

0.06 9.16

0.20 17.40

0.28 15.95

0.0251 19.80 0.0918 0.0056 0.0217 0.13 13.27

a Optimization and frequency calculations were performed at the cp-MP2/aug-cc-pVDZ level; the key parameters computed at the cp-ωB97X-D level are given in parentheses. QTAIM and NBO analysis was performed on the wave functions and orbitals computed at the cp-MP2/aug-cc-pVDZ level. bFrom the dispersed fluorescence spectrum given in Figure S8 (Supporting Information). cForce constants (keff) of the intermolecular stretching normal mode (σ1) or the stiffness of the H-bonds was calculated from computed intermolecular stretches (σ1) by approximating each of the H-bonded fragments as point masses.

(FLP···H2S)+, the computed dissociation energies at the cpB3LYP and cp-ωB97X-D levels were 10.92 and 11.92 kcal mol−1, respectively. The red shift in AIE with respect to monomer, ΔAIE, was calculated from the difference between the D0 values of the cationic and the ground states of the complex. For PHE···H2O, the magnitudes of ΔAIE were 12.93 and 12.73 kcal mol−1 at the cp-B3LYP and cp-ωB97X-D levels, respectively, compared to the experimentally determined value of 13.16 kcal mol−1. The better agreement with the cp-B3LYP value is fortuitous because of the cancellation of errors between the D0 values for the ground and cationic states. We find that

For the cationic state the energies could not be computed at the CBS limit due to the limited computational resource. The cp-B3LYP, cp-ωB97X-D, cp-MP2, and CCSD(T) values for the (PHE−H2O)+ complex were 16.99, 17.91, 15.07, and 16.54 kcal mol−1, respectively. Out of these, the cp-ωB97X-D was found to be the best in agreement with the experimental value of 18.76 kcal mol−1. The cp-B3LYP value was found to be the second best in agreement. The D0 value for the (IND···BEN)+ complex given by the cp-ωB97X-D computation was also found to be in very good agreement with the reported value.48 The D0 values of (FLP···H2O)+ at the cp-B3LYP and cp-ωB97X-D levels were 17.10 and 18.02 kcal mol−1, respectively. For 9391

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the dissociation energies computed at the cp-ωB97X-D level perform consistently better for both the ground state and the cationic state. For the FLP···H2O and FLP···H2S complexes, the magnitudes of ΔAIE were 12.66 and 8.55 kcal mol−1 at the cp-ωB97X-D level, respectively. These numbers can be compared with the experimentally determined values of 12.83 and 8.66 kcal mol−1 for the two complexes. Optimized structures of the complexes at the cp-MP2 and cp-ωB97X-D levels are given in Figures S4−S7 (Supporting Information). A few selected geometrical parameters for the structures optimized at the cp-MP2/aug-cc-pVDZ level, such as the Hbond distance (dH···A (A = O, S)), the distance (RO···A) between the oxygen atom and acceptor, the increase in the phenolic O− H-bond length (ΔrO−H) with respect to the monomer, the Hbond angle (∠OHA (θ)), and the angle (Ψ) made by the angle bisector of ∠HAH in the plane of the acceptor molecule with the phenolic O−H-bond axis are listed in Table 3. For the sake of comparison, a few key parameters obtained at the cpωB97X-D level are also included in Table 3. The complete listing of the x, y, z coordinates for both the complexes at both the computational levels is given in Tables S3−S8 (Supporting Information). The H-bond distance (dH···A) for FLP−H2S in the cationic state was 2.2246 Å, which was about a 12% reduction from that in the ground state. For FLP−H2O the relative reduction in the dH···A was also similar; the H-bond distance in the cationic state was 1.6832 Å. The relative reductions in this parameter at the cp-ωB97X-D level were 12% and 15% for the H2S and H2O complexes, respectively, which is much more in line with the greater stabilization of the H-bond in the cationic state in the latter case. The increase in the phenolic O−H-bond length (ΔrO−H) upon H-bond formation for FLP−H2S in cationic state was 0.0231 Å, which was 4 times higher than that of the ground state of the complex (0.0056 Å). Although in the absolute term this increase was relatively more for FLP−H2O in cationic state, which was 0.0264 Å, the magnitude of elongation was 3 times that in its ground state (0.0079 Å). Although there was no quantitative functional relationship between the H-bond strength and the increase in the O−H-bond length, the increase in the dissociation energy and the O−H-bond length of the respective complexes in the cationic state relative to the ground state were of similar magnitude. The topological parameters of electron densities obtained by the QTAIM analyses using the wave functions generated at the cp-MP2/aug-cc-pVDZ level are given in Table 3, and the molecular graphs showing electron density at bond critical point are shown in Figure 4. There was nearly a 1.6 times increase in the electron density (ρ) and its Laplacian (∇2ρ) at the O−H···A bond critical point (BCP) for FLP−H2O in the cationic state relative to that in the ground state. For FLP−H2S in the cationic state, ρ and ∇2ρ values were 0.0266 and 0.0623 au, respectively, which were almost 2 times its ground state values, which were 0.0136 and 0.0364 au, respectively. All these values are well within the regime of the H-bond.49 Additional BCP was observed between the aromatic ring C−H in the ortho position to phenolic O−H and O/S atom of H2O/H2S in the cationic state. The magnitudes of ρ and ∇2ρ at BCP along the C−H···O H-bond path were found to be 0.0070 and 0.0284 au, respectively, in the case of (FLP−H2O)+; these magnitudes were almost similar to those in the ground state. For FLP−H2S the BCP along the C−H···S was found only in the cationic state; the values of ρ and ∇2ρ at BCP along C−H···S H-bond path were found to be 0.0053 and 0.0170 au, respectively.

Figure 4. Molecular graphs of (a) FLP−H2O and (c) FLP−H2S in the ground state. (b) FLP−H2O and (d) FLP−H2S in cationic states generated from the QTAIM analysis of the wavefunctions optimized at the cp-MP2 level. Red dots are (3, −1) bond critical points.

Although the ρ values at the C−H···A BCPs were much smaller, i.e., only ∼20% of those found for O−H···A, they were within H-bond domain. The ρ and ∇2ρ values were always higher for O−H···O compared to the O−H···S interaction, as listed in Table 3. Natural bonding orbital analysis was performed for the cationic state to obtain second-order perturbative interactions (E(2)(i-j)) between the interacting donor−acceptor orbitals. It was found that for both (FLP− H2O)+ and (FLP−H2S)+, significant interaction involved only one of the lone pairs of the H-bond acceptor and the σ* orbital of phenolic O−H. The second-order perturbative interaction was greater for H2O bound complex (17.40 kcal mol−1) than H2S complex (15.95 kcal mol−1). These values are also compiled in Table 3. Localized molecular orbital energy decomposition analysis (LMOEDA) of the dissociation energy was performed on the geometries optimized at all three different levels such as cpMP2, cp-ωB97X-D, and cp-B3LYP to extract various components of the net LMOEDA interaction (ΔEintLMOEDA) such as dispersion interaction (ΔEintDISP), repulsion (ΔErep), electrostatic (ΔEes), exchange (ΔEex), polarization (ΔEpol), etc. The values of all these interaction components are listed in Table S2 (Supporting Information). At all levels, the H2S complex was found to be predominantly dispersion stabilized in the ground state than the H2O complex, although the percentage of dispersion interaction varied for different methodologies. For FLP−H2S in the ground state, for cpMP2 and cp-ωB97X-D optimized structures, the dispersion contributions were 52.74 and 54.37%, respectively. For the H2O complex in the ground state, the percentages of dispersion contributions were 18.43 and 26.97 for cp-MP2 and cp-ωB97XD optimized geometries, respectively. For FLP−H2S in the cationic state, the percentages of dispersion interactions for cpMP2, cp-ωB97X-D, and cp-B3LYP optimized geometries were 15.38, 23.80, and 15.08, respectively. For FLP−H2O in the cationic state, only 5.77% dispersion stabilization was observed for the cp-ωB97X-D optimized geometry. The percentages of dispersion interactions calculated for cp-MP2 and cp-B3LYP optimized structures turned out to be −6.57 and −0.37, respectively, which is an artifact and the dispersion contribution can be taken as minimal or close to zero percent. 9392

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DISCUSSION The AIE of FLP, FLP−H2O, and FLP−H2S and vibrational levels of their cations were determined by ZEKE spectroscopy. The AIE of FLP was determined as 68 570 cm−1, which was 59 cm−1 smaller than that of phenol (68629 cm−1).44 Therefore, substitution by fluorine at the para position of phenol has a minor effect on the AIE. The AIE of the FLP−H2O and FLP− H2S complexes was determined accurately as 64 082 and 65 542 cm−1, respectively. The red shift in the AIE of the H2O complex was thus 4488 cm−1 with respect to FLP monomer AIE whereas that in the case of the H2S complex was only 3028 cm−1, indicating significantly less stabilization of the complex in the cationic state of the latter. The dominant feature of the ZEKE spectrum of FLP−H2S recorded via S1 000 was the progression along the intermolecular H-bond stretching mode (σ), as shown by red dotted lines in Figure 1. Table 1 lists the intramolecular frequencies observed in the ZEKE spectra and those computed at three different levels, namely, cp-B3LYP, cp-MP2, and cp-ωB97X-D. The comparison shows that the cp-ωB97X-D frequencies are comparable in most cases and better in regard to the intermolecular modes that the other two methods. Similar performance has also been reported recently by Simons et al., who used the B97D+disp functional for predicting the vibrational frequencies of the hydrated complexes of carbohydrates.30 In Table 3, the red shifts in the phenolic O−H stretching frequencies and intermolecular stretching frequencies for both ground and cationic states computed at cp-MP2/aug-cc-pVDZ level are listed along with those computed at the cp-ωB97X-D level in the parentheses. It can be seen that the intermolecular stretching frequencies computed at the cp-ωB97X-D level were in better agreement with the observed frequencies for both the complexes. Higher red shifts in phenolic O−H frequency in the cationic state is indirect evidence of the stronger H-bond compared to the case of the ground state. The intermolecular stretching mode, σ1, of FLP−H2S in the cationic state is identified at 145 cm−1, which is 52 cm−1 higher than that in the S1 state (93 cm−1). It shows that the H-bond force constant increased almost 2.5 times in the cationic state compared with that in the S1 state. This is consistent with the substantial reduction in the equilibrium H-bond distance along the H-bond coordinate of the potential energy surface (Table 3). The observed long progression in this normal mode also indicates a change in the shape of the potential. This is consistent with the energy decomposition analysis, which indicated substantial reduction in the dispersion component in the H-bonding interaction in the cationic state as opposed to that in the neutral ground state. In the S1 state it is not expected to be any different from that in the ground state. Hence the change in the shape of the potential is not surprising. Additionally, it was also observed that the vibrational level spacing between the successive members of the progression in the intermolecular stretching mode was monotonically decreasing, indicating a fair amount of anharmonicity along this normal mode. Because this is the first time the BS extrapolation method was applied to estimate the D0 of H-bonded complex from the spectroscopic data, some validation is in order, which is presented below. The BS method, as applied to the diatomic molecules, assumes that the dissociation products are formed in their ground states, or at least the product states are completely known in order to estimate the dissociation energy. In the

present case what it means is that the equilibrium structures of the individual monomers are not significantly altered in the complex and/or the deformation energy in such cases is negligibly small. In the present case the only change that occurs in the FLP monomer is the elongation of the H-bond-donating O−H-bond by a few milli-angstroms and the deformation energy associated with it is very small, i.e., on the order of a few tenths of a kcal mol−1. Second, in a polyatomic system the 3N − 6 normal modes are invariably mixed with each other. Therefore, in such cases it is important that the bond stretching mode under question be a pure mode or else representing the dissociation coordinate as a simple two-dimensional PE curve would be a gross simplification and inappropriate. At all the levels of theory used in this work the intermolecular stretching normal mode was found to be a pure stretching mode between the FLP and H2S moieties without any mixing with any other intermolecular modes. Therefore, the H-bonded complex can be treated as the pseudo diatomic molecule and the intermolecular stretching normal coordinate can be taken as the dissociation coordinate for the purpose of determining the dissociation energy. For the sake of validation of the D0 estimated using the BS extrapolation, the dissociation energies of the complex cation were also obtained by adding the observed red shift in the AIE of the complex with respect to the monomer to the D0 of complex in the ground state computed at the CBS limit. The D0 for the FLP−H2S complex in the ground state at the CBS limit was computed along with a few other complexes such as PHE−H2O and IND−BEN whose exact values have been determined experimentally as listed in Table 2 to check the authenticity of the CBS values for the FLP−H2S complex. It can be seen from Table 2 for the PHE−H2O complex the D0 computed at the CBS limit (MP2/QZ-TZ) is in best agreement with that determined experimentally.10,48 The D0 computed at the cp-ωB97X-D level is in the next better agreement. It can also be seen that the shift in the AIE, i.e., the ΔAIE value computed using this functional, is also in very good agreement with the experimentally determined number. For the IND−BEN complex, the relatively low-cost cp-ωB97X-D functional computed the D0 in the ground state as 5.29 kcal mol−1, which is only 0.09 kcal mol−1 (∼2%) higher than the experimental value.10,48 It also predicted the D0 for the IND− BEN complex in its cationic state to within 5% of the experimental value. The difference between these two energies, i.e., the shift in the AIE, was 7.14 kcal mol−1, which is in excellent agreement with the experimental measurement.48 For the FLP−H2S complex in the ground state, the D0 value at the CBS limit at the MP2/QZ-TZ level was found to be 3.50 kcal mol−1 and that computed using the cp-ωB97X-D functional was close to it, i.e., 3.37 kcal mol−1. The D0 value in the cationic state was computed as 11.92 kcal mol−1 at the cp-ωB97X-D level. The dissociation energy difference in the ground state and the cationic state gave the ΔAIE of 8.55 kcal mol−1 in excellent agreement with the experimental value of 8.66 kcal mol−1. Therefore, on the basis of the ability of this functional (cpωB97X-D) to reproduce the dissociation energies of the reference complexes in their ground states and those computed at the CBS limit using MP2/QZ-TZ extrapolation, and the ability to predict the ΔAIE in good agreement with the experimental value, we strongly believe that the D0 value calculated using this functional as 11.92 kcal mol−1 to be close to the true dissociation energy of the (FLP−H2S)+ complex. 9393

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The D0 obtained by the BS linear extrapolation is about ∼18% lower than the best computed value. Although there are many methods for determining the heats of dissociation of molecules that are stable at room temperature, for the weakly bound complexes this is not always feasible. Therefore, the BS extrapolation method seems like a simple method to estimate the dissociation energies if the method is validated properly. A critical analysis of the BS extrapolation has been provided by Gaydon in his classic paper.47 As long as the rate of convergence of the vibrational levels is same throughout the PES up to the dissociation limit, then the BS extrapolation is expected to give reliable estimates. In reality, however, this statement is not totally correct due to the fact that all higher order terms in the expression for the vibrational energy levels have been ignored. These terms become more effective at large ν levels, if the magnitudes of the higher order parameters are significant, and will cause the negative deviation in the linear extrapolation, making the BS extrapolated values larger than the true values. On the other hand, Gaydon had pointed out another aspect, namely, at long interatomic distances if the potential varies as inverse power of the distance where the power could vary from 3 to 6 depending on the forces operating between the two fragments, then the potential does not reach the dissociation limit exponentially but much slower than that.47 This would cause a decrease in the rate of convergence of the vibrational levels and will make the BS estimate lower than the true dissociation energy. The LMOEDA analysis of the interaction energy shows (Table S2, Supporting Information) that even in the cationic state of FLP−H2S, there is 24% dispersion stabilization for the structure optimized at cp-ωB97X-D level. The dispersion forces or the van der Waals forces have a greater range than the exponentially decreasing valence forces. It has been pointed out47 that in some cases near the point of convergence a slight bending up of the Δε curve may be expected on account of the van der Waals forces. Second, in the case of singly ionized molecules, the dissociation takes place into one neutral and one charged species. In such cases the force between the two fragments at large distance will vary as 1/r4. Therefore, in the present context both these factors could make the convergence of the levels slower at higher ν levels, giving rise to the positive curvature to the linear plot. In such cases, the actual D0 is expected to be higher than its value estimated from the BS extrapolation. Therefore, the 18% lower value obtained by the BS extrapolation compared to the best computed value does not come as a surprise. In the same paper, Gaydon47 pointed out that the error in the BS estimation relative to the true dissociation values depends on the class of molecules and it was also shown that for the ionized molecules that dissociate into one charged and one neutral species the BS estimated values were in good agreement (within 15%) with the true dissociation energies and this was encouraging for us. For the FLP−H2O complex also the dominant feature of the ZEKE spectrum (Figure 2) was the progression along intermolecular H-bond stretching mode (shown in red dotted lines) when excited via S1 000. The intermolecular stretching mode for FLP−H2O in the cationic state is found at 235 cm−1, which is higher than that in the S1 state (160 cm−1); i.e., there is a factor of 2 increase in the force constant relative to that in the S1 state. The red shift in AIE of FLP−H2O is much higher than that in FLP−H2S, which shows the stabilization of the FLP− H2O H-bond in the cationic state with respect to the ground state is higher than that in the case of FLP−H2S. This can be

argued qualitatively by realizing that since the interaction is largely electrostatic in nature in the cationic state, only the electrostatic part of the H-bond interaction in the neutral complex would get enhanced. In the case of FLP−H2O, the neutral ground state complex, the H-bonding interaction is predominantly electrostatic in nature whereas in FLP−H2S the electrostatic component is much smaller; i.e., it is mostly dispersion stabilized (>50%) because of the poor electronegativity and high polarizability of S atom. Unlike the H2S complex, in the case of FLP−H2O, the progression members of the intermolecular stretching vibration were equally spaced, indicating that along this normal coordinate the potential is quite harmonic within the observed five quanta of transitions. This has also been observed in the case of ZEKE spectra for other analogous complexes such as phenol−water45 and hydroquinone−water.50 The net dissociation energies of the water complexes with phenol derivatives are on the order of 18−19 kcal mol−1. The almost harmonic nature of the σ normal coordinate in this case may be due to the deeper and narrower potential well. This can be contrasted with that of the FLP−H2S complex where the well depth is on the order of ∼12 kcal mol−1. The shallow well depth of the O− H···S interaction relative to that of the O−H···O interaction and the significant change in the character of the PES in the O−H···S complex could be the reason for the discernible anharmonicity along the intermolecular stretching mode in the case of FLP−H2S complex. This is an interesting observation and needs to be validated further by investigating more examples of O−H···S bound complexes. Finally, it must be emphasized that although there are a few methods to determine the binding energies of the weakly bound complexes as outlined in the Introduction, not all the complexes are amenable to the application of these methods. It largely depends on the relative energies of the S1 state, S1−D0 energy gap, and the relative ionization potential of the monomers. On this backdrop we have demonstrated that that the BS extrapolation can give reliable estimates of the binding energies provided one can observe the anharmonicity in the first few vibrational levels along the dissociative coordinate.

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CONCLUDING REMARKS The FLP monomer and the FLP−H2S and FLP−H2O complexes were characterized in the cationic state by ZEKE spectroscopy. For the first time, the S-centered H-bonded complex was characterized in the cationic state. The AIE values of FLP, FLP−H2O, and FLP−H2S were determined to be 68 570, 64 082, and 65 542 cm−1, respectively. The red shift in AIE for FLP−H2S was 1460 cm−1 (4.17 kcal mol−1), lower than that for FLP−H2O, which indicated that the H2S complex was less stabilized in the cationic state with respect the ground state compared to that for the H2O complex. The intermolecular stretching frequencies and its overtones were observed in the ZEKE spectra for both complexes. Within the observed quanta of transitions, the intermolecular stretching potential seemed harmonic for FLP−H2O in the cationic state; however, for FLP−H2S in the cationic state, significant anharmonicity was observed along the H-bond stretching coordinate. From the anharmonicity constant of FLP−H2S, the D0 in the cationic state was estimated as 9.72 ± 1.05 kcal mol−1 using BS extrapolation. The application of the BS extrapolation to the Hbonded complex was validated with the help of ab initio computations, and it was established that the BS estimated value was about 18% lower than the best computed value of 9394

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11.92 kcal mol−1 at the cp-ωB97X-D level. It was also established that the ωB97X-D functional was better in computing the dissociation energies in the ground state as well as in the cationic state. It also did a fairly good job in computing the vibrational frequencies in the cationic state. In summary, we strongly feel that this simple method can be used to determine the dissociation energies of weakly bound complexes provided that the intermolecular stretching mode is a pure local mode and a sufficient number of levels can be observed experimentally with reasonable anharmonicity.

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ASSOCIATED CONTENT

S Supporting Information *

The PIE curves and ZEKE spectra of FLP, FLP−H2O, and FLP−H2S by exciting via the S1 000 intermediate, minimum energy conformations, dispersed fluoroscence spectra, listing and assignments of all observed transitions in the ZEKE spectra, energy decomposition analysis, optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*S. Wategaonkar. E-mail: [email protected]. Phone: +91-222278-2259. Present Address †

IAMS, Academia Sinica, P.O. Box 23-166, Taipei, Taiwan.

Notes

The authors declare no competing financial interest.

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REFERENCES

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